1. Field of the Invention
The present invention pertains to torque limiting devices and torque limiting devices with trip indicators.
2. Background Information
The present invention is well adapted for use in aircraft flight controls, such as trailing edge flap systems where a plurality of torque responsive elements are driven from a single drive source. However, the invention can be practiced in any system where the drive unit must be prevented from exerting excessive torque.
A typical form of the prior art wing flap torque lockout system is shown in U.S. Pat. No. 4,030,578 to Cacciola et al. In that patent, the device operates by transmitting torque between two axial members through a ball-ramp coupling. The ball-ramp coupling comprises a plurality of balls interposed between the two axial members, with the balls engaged within precisely shaped sockets contained within each axial member. As excess torque occurs, the plurality of balls cause one member to axially displace with respect to the other member compressing a disc brake pack allowing the excess torque to be reacted in the housing, thereby preventing torque transmission. The disc brake pack is comprised of alternating stationary and rotating friction plates.
While this torque limiter has been used successfully on many applications, it is greatly affected by the friction coefficient of the disc brake members and the viscous drag torque, which is related to the plurality of plates, lubricating medium and the rotational speed. This viscous drag torque of the disc brake members causes a significant loss of drive system efficiency and increases drive system weight.
Disc brake packs have been used for many applications because the operating speeds exceed the engagement speed for which toothed, or jaw clutches should be used. At higher operating speeds, a disc brake pack is typically used. Trailing edge flap systems typically operate at speeds in excess of 400 rpm.
One attempt to eliminate the disc brake pack and the adverse effects associated with it is disclosed in U.S. Pat. No. 5,299,666 to Lang et al. In this patent, the device operates by transmitting torque between two axial members, a drive member and a driven member, having a plurality of balls therebetween. The plurality of balls engage sockets contained within each of the axial members. As excess torque occurs, the plurality of balls causes the drive member to axially displace with respect to the driven member. A first set of teeth contained on the drive member engage with a second set of teeth aligned to the first set of teeth and fixed to the housing to prevent torque transmission.
The problem with this device stems from the fact that the first set of teeth is contained on the drive member. The placement of the first set of teeth on the drive member prevents the torque produced by the contact of the teeth from being transmitted across the plurality of balls. Consequently, no additional axial force is developed by the ball-ramp coupling to aid the engagement of the teeth. To solve this problem, the teeth are required to have a negative pressure angle so that they can pull themselves into engagement. The negative pressure angle on the teeth is expensive to manufacture and is not suitably robust for continued service in many aircraft drive systems. Any degradation to the edges of the teeth caused by repeated use will cause the teeth not to engage upon contact, and will lead to excessive torque output. Additionally, the negative pressure angle on the teeth makes it necessary to slowly reverse the direction of the drive system to allow the brake to disengage. Many drive systems require a torque limiting device that disengages automatically, upon removal of input torque.
It is also common to provide a trip indicator that indicates when the torque limiter has been activated. Modern aircraft have multiple torque limiting brake mechanisms. The trip indicators are used for troubleshooting purposes, to determine which torque limiter has activated. Many of the prior art trip indicators produce false trip indications, meaning a trip is indicated when the torque limiter has not been activated. Most prior art trip indicators utilize the axial motion of the output cam to actuate the indicator. It is possible to have a small amount of axial motion of the output cam trip the indicator but not energize the braking mechanism. This produces a false trip indication. A false trip indication makes troubleshooting the drive system difficult and can lead to unnecessary maintenance activity.